1. Field of Invention
The present invention relates to telecommunications. More particularly, the present invention relates to telecommunications with a phase shift in at least one path.
2. Description of Related Art
FIG. 1A shows a conventional direct-down tuner. FIG. 1B shows frequency spectrums of the signals 101, 103, 105, and 107 respectively. The direct-down tuner uses tracking filters (102 and 104) to eliminate the frequency components of the RF signal 101 outside the desired channel (channel 39). A local oscillator 106 (LO) generates a reference signal at 372 MHz. The mixer 108 mixes the reference signal with the filtered RF signal 105 so that the filtered RF signal 105 is down-converted to an IF (intermediate frequency) signal 107. The desired IF band 110 is from 54 MHz to 60 MHz. The central frequency of the IF band 110 is 57 MHz. However, the frequency components of the RF signal 101 in the image channel (channel 58) cannot be completely suppressed or removed by the tracking filters 102 and 104, which results an interference from the image channel into the desired channel. Further, it is also difficult to integrate the tracking filters with other elements of the conventional tuner into a single chip.
FIG. 2A shows a conventional up-down tuner. FIG. 2B shows frequency spectrums of the signals 203, 207, 213, and 215 respectively. The RF signal 201 is up-converted by a mixer 205 cooperating with a local oscillator 212. The up-converted RF signal 207 is sent to a band-pass filter 204 (BPF). The mixer 206 cooperates with a local oscillator 214 down-converts the 213 to an IF signal 215. There is no image channel corresponding to the desired channel since the RF signal is up-converted before being down-converted to the IF signal. However, the conventional tuner in FIG. 2A involves high frequency operation (about 1 G Hz).